This invention relates to the operation of heart pacemakers. It has particular relationship to operation of pacemakers having leads or electrodes both to the atrium and to the ventricle and operating in the DDD mode. During normal operation, the P wave is produced as a result of contraction of the atrium and a QRS wave as a result of contraction of the ventricle. The expression DDD is a code which identifies a mode of operation of pacemaker. Generally, D means double. The code DDD is part of a system of codes describing the operation of pacemakers which have been developed over a period of years. Among the publications dealing with these codes are Parsonnet, V., Furman, S., Smyth, N.P.D.: Report of the Inter-Society Commission for Heart Disease Recourses--Implantable Cardiac Pacemakers: Status Report and Resource Guidelines, American Journal of Cardiology, Vol. 34, pp. 487-500, Oct. 1974 and V. Parsonnet, S. Furman and N. P. D. Symth--A Revised Code for Pacemaker Identification: PACE, Vol. 4, pp. 400-402, July-August 1981. Victor Parsonnet Chairman, Seymour Furman, Nicholas P. D. Smyth, Michael Bilitch Members--Report of Inter-Society Commission for Heart Disease Resources--Optional Resources for Implantable Pacemakers--American Heart Association--Circulation, Vol. 68, pp. 227A-244A, July 1983. For the three-letter code, the first letter signifies what chamber is paced, the second what chamber is sensed for naturally occurring waves, and the third, the response of the pacemaker to the sensed wave; either triggered (T), inhibitory (I), or both (D).
With respect to the code DDD, the first D means that both chambers, the atrium and the ventricle, are paced, the second D means that both chambers are sensed for naturally-occurring or intrinsic waves and the third D means that the packemaker responds to the sensing by both triggered and inhibitory responses. If the pacemaker senses an intrinsic wave in the atrium, it triggers a pulse for the ventricle which is delivered after the atrio-ventricular (AV) delay. If, in the meantime, a ventricular wave is sensed, the ventricular pulse which was triggered in the atrium is inhibited in the ventricle. If an intrinsic atrial wave is not sensed, the pacemaker produces an atrial pulse and after the AV delay, the pacemaker produces a ventricular pulse. If, in this case, an intrinsic ventricular wave is sensed, the pacemaker pulse is inhibited. This may occur even for a wave occurring as a premature ventricular contraction (PVC).
The atrial or ventricular pulse from the pacemaker has the appearance in an ECG of a sharp line or spike and is frequently referred to herein as a "spike" . Usually an intrinsic event will be referred to herein as a wave and the output signal to the heart of a pacemaker will be referred to herein as a spike or pulse. A spike to the atrium stimulates a P wave, a spike to the ventricle stimulates a QRS wave. A spike, a P wave or a QRS wave is sometime referred to herein as an event.
At this point, it appears desirable to discuss the frequency or period of heart beats and related intervals. The typical heart beat frequency in beats per minute of a person who is resting is about 70; the corresponding period is 857 ms. A person who is exerting himself, running or exercising vigorously, may have 160 heart beats per minute; the corresponding period is 375 ms. Pacemakers are normally programmed to operate at about 70 beats per minute. When an intrinsic atrial wave occurs, it triggers the pacemaker to produce a ventricular pulse which may be inhibited by the occurrence of an intrinsic ventricular wave. In the absense of intrinsic waves during an interval, the pacemaker would produce atrial pulses at the frequency for which it is programmed and ventricular pulses would be triggered after each AV delay. The AV delay is a pacemaker interval which is programmed. It ranges up to 250 ms, but is typically set in the range between 125 ms and 200 ms.
This invention concerns itself with pacer mediated tachycardia (PMT) which has complicated the application of physiological pacing in the DDD mode with a pacemaker which has leads both to the atrium and to the ventricle. A patient who encounters pacer mediated tachycardia experiences a rapid heat-beat, which not only causes the patient discomfort, but may also be hazardous.
The cause of PMT is retrograde conduction; i.e., conduction from the ventricle to the atrium (VA conduction). Of patients with complete antegrade atrio-ventricular (AV) block, about 30 to 40% have intact retrograde conduction. Of patients having intact; i.e., normal AV conduction, about 80 to 85% have intact retrograte; i.e., VA conduction. A patient may have intact AV conduction and may still require a pacemaker because his heart beat is at a low frequency. In a pacemaker operating in the DDD mode, a paced beat or an intrinsic beat in the atrium triggers a beat in the ventricle. These ventricular beats may generate a P wave in the atrium by retrograde conduction. More commonly, the P wave may be generated by retrograde conduction by the electrical pulse resulting from a premature ventricular contraction (PVC). The generation of the retrograde P wave by a paced pulse or normal intrinsic ventricular wave is less likely because when the paced pulse or normal intrinsic wave is generated, the retrograde conduction path may be closed, while when the PVC wave is generated, the path is open or is likely to be open. Essentially, the retrograde P wave is a feedback impulse. It causes the pacemaker to fire to the ventricle after a fixed or programmed AV delay. The resulting ventricular depolarization sends a retrograde impulse to the atrium repeating the pulsing similarly to the operation which takes place in a feedback oscillator. The resulting PMT usually causes the pacemaker, at once, to produce pulses at the upper rate limit of the pacemaker. This upper rate liimit is timed by an additional timer in the pacemaker maker which is progammable. It is dependent on the AV delay and on the post ventricular atrial refractory period (PVARP), and may have any magnitude from about 90 to 170 beats per minute. It limits the rate or frequency beats at which the pacemaker can operate. What happens is that after each excitation of a P wave by retrograte conduction, the high-frequency timer and the AV delay time out and when they have both timed out, a ventricular pulse is fired and the process is repeated. The high-frequency timer precludes pulsing at the periodicity of the AV delay which may exceed 300 pulses per minute. The resulting high beat frequency discomforts the patient and, in addition, subjects the patient to risk. The PMT may be stopped by fatigue in the retrograde path, or it may be stopped by application of a magnetic signal to convert the pacemaker to fixed pacing in the DOO mode, temporarily. In the DOO mode, both chambers are paced, but there is neither sensing of the chambers nor, in the absence of sensing, response of the pacemaker in either the triggered or inhibitory mode.
In accordance with the teachings of the prior art, PMT is precluded by elongating the postventricular-atrial refractory period (PVARP) for one period responsive to the sensing of a premature ventricular contraction (PVC). The PVARP is lengthened for only one period; i.e., the one after the PVC occurs. The refractory period is the period during which the pacer cannot "see" an intrinsic wave. By elongating the PVARP, the sensing of a P wave generated in the atrium by feedback is blocked so that PMT is precluded. The teaching of the prior art is to lengthen the PVARP to about 340 ms. The principle disadvantage of this practice is that this lengthening of the PVARP is frequently inadequate. Furthermore, the lengthening of one PVARP for only one period does not preclude recurrence of PMT even if the tendency for it to occur was precluded this once. In addition, this practice does not resolve the case in which a PMT is triggered by a paced pulse or normal intrinsic ventricular wave.
Another attempt to solve the PMT problem is to shorten the atrio-ventricular delay and to set the upper rate or frequency limit at a high level. The resulting enhancement of the likelihood of fatigue is relied upon to stop the PMT. This "cure" appears worse than the "disease".
It has also been proposed that after the PMT is experienced, the physician should elongate the post-ventricular atrial refractory period (PVARP) so that it exceeds the retrograde ventricular-atrial (VA) conduction time. To accomplish this object, the physician programs the pacemaker for a PVARP longer than the anticipated VA conduction time. It is anticipated that the pacemaker then will not respond to the feedback pulse and PMT will be precluded. In some cases, the retrograde VA conduction time is of relatively long duration. Durations as long as 460 to 480 ms have been experienced. It is then necessary, so as to cover all cases, that the pacemaker have the facility to program the PVARP to durations of between 500 and 550 ms and to maintain this function at that duration to suppress or prevent PMT. This alternative, which is put into effect only after the tachycardia is discovered, has the disadvantage that until the patient visits his or her physician, the PMT is not discovered and he or she is subject to the discomfort, distress and hazard of an abnormally rapid heart beat.
Another alternative has been proposed by Intermedics, Inc., P.O. Box 617, 240 Tarpon Inn Village, Freeport, Tex. 77541. the PMT beats occur at precise intervals as compared to sinus tachycardia, the increase in beat rate by reason of exertion. In the practice of the Intermedics method, a predetermined number, typically 15 beats are sampled. If the sampled beats are regular with unchanging intervals between them, PMT is recognized. The Intermedics practice in dealing with PMT, once it is detected, is to block or drop the next ventricular paced beat. The PMT is interrupted. The disadvantage of this method is that it does not prevent the recurrence of PMT.
It is an object of this invention to overcome the above-described drawbacks and disadvantages of the prior art and to provide a method of operating a pacemaker, having leads to both the atrium and ventricle and normally operating in the DDD mode, in whose practice PMT with its discomfort and hazard to the patient shall be automatically detected and automatically suppressed and prevented from recurring before the host visits a physician and in addition, the physican will be made aware of these events.